1. Field of the Invention
This invention relates to a liquid crystal display device having a liquid crystal layer in which liquid crystal molecules are oriented in a twisted structure and more particularly it relates to a liquid crystal display device in which liquid crystal molecules are oriented stably in a state suited for high speed operation even if the twisted structure has a large twist angle.
2. Description of the Prior Art
U.S. Pat. No. 3,918,796 discloses a twist nematic liquid crystal display device in which liquid crystal molecules are oriented to have a twisted structure of 90.degree.. Such a liquid crystal display device having 90.degree. twisted orientation has however a narrow viewing cone and is not suited for high level multiplexing drive. For example, in a display device of a matrix type, the contrast is decreased as the multiplexing ratio is increased, and when the display device is driven by a multiplexing ratio of 1/100, the contrast value becomes 3 to 5, which value can not put the device to practical use. Moreover, this contrast value is a value in case of viewing the display from a specified direction and the range of the visual angle is limited to about 20.degree. or less in any direction with respect to a line perpendicular to the display face. Consequently, it follows that such a liquid crystal display device can not be applied to a large-area display (such as dot matrix display having a high resolution).
In view of the above described situation, studies were pursued as to the twist angle of liquid crystal. As a result, a liquid crystal display device using another display mode was proposed as disclosed in European Patent Application Laying-Open Gazette EPO-131,216. In this liquid crystal display device, the twist angle of a liquid crystal layer is made as large as 160.degree. to 360.degree. and by utilizing the birefringence of liquid crystal molecules, a practically applicable contrast value can be obtained even if the display device is driven at a multiplexing ratio of 1/100 to 1/200.
However, for a liquid crystal display device utilizing the above described display mode, it is required to make stable the orientation of liquid crystal molecules (namely, to take measures for undesirable domain and measures for pretilt angle control). To take measures for undesirable domain is to prevent lack of uniformity in the orientation due to increase of a twist angle. For example, if the liquid crystal molecules are forced to have a clockwise orientation, an anticlockwise orientation or an orientation of a different twist angle is partially occurred. When such phenomenon occurs, there is caused an ununiformity in the colors in the non-operated state or the response time in the operated state of the display device, resulting in a considerable decrease in the quality of display. On the other hand, to take measures for pretilt angle control is to prevent slow-down in the response time due to a low pretilt angle formed by liquid crystal molecules with respect to a substrate surface. Although a high molecule orientation film used for 90.degree. twisted orientation has a pretilt angle of 0.degree. to 2.degree. (low pretilt angle), a pretilt angle in this display mode is required to be 5.degree. or more, preferably 10.degree. to 30.degree. (high pretilt angle). Further, in case of a low pretilt angle in the above described mode, undesirable domains are liable to occur due to application of the threshold value.
In this regard, the above stated EPO-131,216 uses an orientation film obtained by evaporating an inorganic material obliquely onto a substrate so that a high pretilt angle orientation is obtained. However, since the incidence direction of evaporation is an important factor in making such an orientation film, such an orientation film obtained as a uniform film has a limited area, i.e., an area of only 10cm.sup.2 at most. In addition, an acceptable allowance of thickness of a liquid crystal layer is small in this display mode and as a result if the thickness of the orientation layer lacks uniformity, this display device can not be used. For those reasons, such an oblique evaporation method is not suitable for production of a large-area display device or mass production of such display devices.